The present invention relates to digital signal processing and filtering and, more particularly, to a digital filter and filtering method that attenuates the noise content of sampled analog signals.
Control systems are designed to regulate the performance of a device or physical parameter to a particular configuration or value. A control system may include an input to receive a command signal representative of a desired configuration or value, and an output that supplies a process control signal that is used to regulate the device or parameter to achieve that desired configuration or value. In a close-loop control system, a sensor may be used to sense the device configuration or parameter value, and supply a feedback signal to the control system input. The command signal and the feedback signal may then be compared, and the process control signal appropriately adjusted until the input signal and feedback signal comparison indicate that the device or parameter has achieved the desired configuration or value.
The control systems generally described above may be implemented using either an analog based design, a digital signal processor (DSP) based design, or a combination of both. The performance of a control system, no matter how it is implemented, may be adversely affected by noise. Noise may be injected into the control system from various sources, including electromagnetic interference (EMI), poor system grounds, ground loops, and the sensors used to provide the feedback control signals. The magnitude of the noise injected into a control system from these various sources can be controlled to a certain extent. However, injected noise can never be fully eliminated. Thus, the remaining noise that is injected may either be filtered out or tolerated in the system design.
In analog systems, noise filtration may be provided by incorporating one or more filter circuits, which generally consist of discrete components. In DSP based systems, digital filters are incorporated to attenuate injected noise. Digital filters are processors programmed to implement an algorithm that performs predetermined numerical calculations on sampled values of a signal.
No matter which type of filtration is used, analog or digital, the filter's effectiveness against noise may present a trade off with control system performance. In other words, as a filter's effectiveness against noise increases, the control system's performance may decrease. This is because increasing a filter's noise attenuating effectiveness may add phase lag to the filtered control signals, which in turn may reduce control system stability. To accommodate reduced control system stability, the bandwidth of the system may be reduced, which in turn may decrease control system performance. Moreover, in digital based control systems, using digital filters to attenuate noise may increase computation time of the processor(s). Although digital filters have been designed that minimally impact control system performance, these filters are generally high order filters with potentially significant computation time demands.
Hence, there is a need for a digital filter that can effectively attenuate noise in a control system while having minimal impact on system performance and that does not require significant computation time. The present invention addresses this need.